A hulking and twisted distortion of Spider-Man, Venom is the result of an alien symbiote merged with a human holding a bitter grudge against Peter Parker. This combination has proven nearly lethal to the wall-crawler numerous times.
Despite his hatred of Spider-Man, Venom has a twisted sense of morality, avoiding harming anyone he deems to be innocent and even performing heroic deeds. However, it is never clear how long Venom's darker impulses can be held in check. No matter how hard anyone tries to keep the symbiote down, Venom always manages to come back, like a plague.
A Race of Symbiotes
Venom's origins can be traced to a race of symbiotes from the planet Klyntar. These beings merge with hosts, and the combined entity tends to have a personality based on both the symbiote and the host itself, though sometimes one side or the other tends to dominate.
One particular symbiote found itself accidentally part of the scenery of Battleworld, a pieced-together, alien world created by the Beyonder to test heroes and villains against one another, including Peter Parker, AKA Spider-Man. After a pitched battle, Spider-Man's costume was ripped to shreds, and his ally Thor mentioned a device that restored fabric.
Instead, Spider-Man inadvertently used a different device, which freed the symbiote. It immediately bonded to Spider-Man, providing him a sleek black costume and giving him notable new abilities. Instead of Peter Parker's web shooters, Spider-Man's webbing was now generated from the costume. In actuality, parts of the symbiote itself and never ran out. On top of that, his reflexes were even better than before, and Spidey could now morph the suit into any clothing he wished.